1. Field of the Invention
The present invention relates to a semiconductor device, especially, a high-frequency output transistor and, more particularly, to an emitter stabilizing resistor connected to an emitter region of the transistor stabilizing the transistor and preventing damage to the transistor upon overloading.
2. Description of the Related Art
A so-called emitter stabilizing resistor (ESR) (or emitter series resistor) is disclosed, e.g., in Japanese Patent Publication Sho 52-3787. The ESR is formed by, e.g., an impurity layer formed in a semiconductor substrate. The ESR is used to improve the stability of a transistor a signal having a large magnitude is applied and to increase a breakdown voltage. The ESR requires the sheet resistance of greater than 1 to less than 100 .OMEGA./.quadrature.. However, when the ESR is formed by an impurity layer, in order to obtain this sheet resistance, the impurity layer must be formed by a special process. In addition, the ESR may be formed by, e.g., a metal thin film in place of a diffusion layer. However, the metal thin film is not preferable because the sheet resistance of the ESR formed by the metal thin film may generally only be reduced to 10 .OMEGA./.quadrature. due to limitations of the material. As a method of decreasing the resistivity of the ESR, it is effective to increase the diameter of a lead hole of the resistor. However, when the diameter of the lead hole is increased, the size of the entire ESR is also increased. Therefore, parasitic capacitance is increased, and the capacitance connected to the emitter in parallel is increased. This parasitic capacitance degrades the high-frequency operation of the transistor.
As described above, in the prior art, it is difficult to reduce the resistivity of the ESR while retaining good high-frequency characteristics.